The main physiological role of white adipose tissue (WAT) is to supply energy when it is needed by other tissues. In mammals, white adipose tissue is the primary energy storage depot, accumulating fuel reserves in the form of triacylglycerol (TAG) during times of energy excess. The release of free fatty acids (FFA) from TAG is stimulated by catecholamines and regulated by hormones such as insulin, glucagon and epinephrine. The most important enzyme in WAT believed responsible for hormone regulated hydrolysis of triglyceride is hormone sensitive lipase (HSL).
Dysregulation of adipocyte lipolysis, resulting in elevated circulating non-esterified fatty acids (NEFA) is associated with obesity and co-morbidities including the development of type 2 diabetes. Obese or insulin resistant subjects have increased visceral adipose tissue depots. These depots contain elevated levels of HSL protein and exhibit enhanced lipolytic activity as they are resistant to the insulin-mediated suppression of lipolysis. This results in increased plasma levels of free fatty acids (FFA), which further exacerbates insulin resistance due to the accumulation of triglycerides in tissues other than WAT such as liver, pancreas and muscle. Thus, the elevated plasma levels of FFA due to increased HSL activity contributes to and worsens insulin resistance in obese and type 2 diabetic individuals. Restoring the exaggerated plasma FFA and triglyceride levels through inhibition of HSL would reduce the accumulation of triglycerides in tissues other than WAT, such as liver, muscle and the pancreas resulting in decreased hepatic glucose output, increased muscle fatty acid oxidation and improving β-cell function.
Elevated FFAs are also associated with increased cardiovascular risk, including atherosclerosis and myocardial dysfunction. Furthermore, high lipolytic activity and elevated FFAs lead to increased insulin resistance and hypertension in hypertensive rats. The FFA collect in the liver and lead to increased production of TAG, which are packaged into very low density lipoproteins (VLDL) which are secreted. Therefore, reducing the activity of HSL would decrease the release of FFA to the blood, thus limiting the supply of FFA to the liver for TAG synthesis. Thus, HSL inhibitors could have beneficial effects as treatment of non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH).